Theory of optical nonlinearities in semiconductors: Applications to nonlinear wavemixing and photonic switching.

摘要

Nonlinear optical switching and grating formation/scattering effects in thin semiconductor samples are modeled utilizing a combined microscopic/macroscopic theoretical approach. The microscopic optical nonlinearities of the materials of interest are treated using the semiconductor plasma theory of Banyai and Koch. Computer simulations are used to evaluate the models developed under both steady-state and dynamic conditions. The overall objective of the simulations is to provide a realistic assessment of the potential performance of semiconductor etalon-based nonlinear optical devices for photonic switching and processing applications. To this end, parameter studies are performed with the goal of finding device designs that can operate cascadably, at subnanosecond speeds, with minimum switching energy. Both bistable and two-wavelength operating modes are investigated, with an emphasis on dynamic determination of system characteristics such as contrast, fanout, and differential gain. For bistable semiconductor etalons, the maximum gain achieveable is found to depend in a fundamental way on the relationship between the pulsed-excitation timescale t(p) and the carrier lifetime τ(R). Significant differential gain is shown to disappear for GaAs etalons as t(p)/τ(R) approaches unity, implying that subnanosecond cascading of bistable devices is impractical in this case. For two-wavelength logic gates, several potential solutions to the well-known input/output-wavelength incompatibility problem are proposed. Through the use of a NOR-gate/upconverter etalon pair, picosecond cascadable operation with a fanout of two and contrast of at least five are predicted, requiring a total input energy of 75 picojoules. Utilizing an injected current and stimulated recombination in an active NOR-gate design, the total input energy can be reduced to about 25 picojoules for cascadable, high-contrast operation. The nonlinear semiconductor field-propagation model is also applied to the case of degenerate four-wave mixing in the Raman-Nath regime. The resulting theoretical framework is compared with widely-used small-signal analyses of DFWM in semiconductors for the case of bulk GaAs. The comparison makes clear the inadequacies of such approaches in extracting nonlinear material properties from DFWM experiments performed using moderate to high input intensities. Dynamic simulations of diffraction efficiency spectra in low-temperature CdS are compared with data from corresponding pulsed experiments, producing good qualitative agreement. On the basis of the observed theory-experiment correlation, several drawbacks of DFWM spectroscopy in comparison to pump-probe techniques are discussed.